1. Field of the Invention
The present invention relates to a camera, a lens apparatus, and a camera system which are provided with an autofocus mechanism, and more particularly, to those which use a vibrating type motor to drive focusing in an image-taking optical system.
2. Description of the Related Art
A vibrating type motor, also referred to as an vibration wave motor or the like, produces output in such a manner that two-phase cycle signals with different phases are applied to electro-mechanical energy conversion elements (electrostrictive elements) provided for a vibrating member to cause the vibrating member to vibrate in a traveling wave (that is, to generate a traveling vibration wave on a surface of the vibrating member), thereby driving a moving (driven) member in press contact with the vibrating member by friction. In the vibrating type motor, the vibrating member is made non-vibrating to hold the position of the moving member by a frictional force acting between the vibrating member and the moving member.
Since such a vibrating type motor is characterized by high torque at low rotational speed, driving noise hardly produced, favorable responsiveness and the ability to accurately control positions, it is used for autofocus drive in cameras, interchangeable lenses or the like.
For autofocus (AF) schemes of cameras or the like, an AF system employing a phase difference detection scheme is used in many models of cameras of a so-called single-lens reflex camera type in which images are taken on silver films. The AF system of the phase difference detection scheme operates as follows.
As shown in FIG. 12, light flux incident through an image-taking lens is reflected to a lower portion of a camera by a sub-mirror 502 attached to the back of a semi-transparent main mirror 501 disposed at an angle of 45 degrees to an image-taking optical axis L. The reflected light flux passes through an infrared cutting filter 504, and is divided into two parts by a field lens 503 of a secondary optical system. The two parts of light flux form two images on a pair of AF sensors in an AF sensor unit 505 through a secondary imaging lens 508.
The paired AF sensors 506, 507 are disposed side by side and produce outputs as shown in FIG. 13. A difference in spacing between the outputs from the two images formed on the paired AF sensors 506, 507 is relied on to determine an in-focus, a front-focus, or a back-focus state. Focusing is achieved by moving a focus lens such that the spacing between the outputs of the images matches the spacing in the in-focus state.
The amount of the movement of the focus lens, that is, the amount of movement of an image surface, is determined by calculation from the spacing between the outputs of the two images with the following algorithm.
First, the outputs from the two AF sensors 506, 507 are acquired as data, and the correlation is examined between the outputs from the two sensors 506, 507. The correlation is determined with “the MIN algorithm” in which a correlation U0 is calculated as:
  U0  =            ∑      j      m        ⁢          min      ⁡              (                              A            ⁡                          [              j              ]                                ,                      B            ⁡                          [              j              ]                                      )            
(min(a,b) refers to a smaller one of a and b) where data from the sensor 1 (506) is represented by A[1]–A[n] and data from the sensor 2 (507) is represented by B[1]–B[n].
After the calculation of the U0, a correlation U1 is calculated between data on an image A shifted by one bit in the AF sensor and the data on an image B as shown in FIG. 14. The U1 is represented as:
  U1  =            ∑      j      m        ⁢          min      ⁡              (                              A            ⁡                          [                              j                +                1                            ]                                ,                      B            ⁡                          [              j              ]                                      )            
Correlations are successively calculated from images shifted on a bit-by-bit basis. The correlation is at a maximum value when two images match, and the amount of shift at the maximum value is found. From data in the neighborhood of the value, a true maximum value of the correlation is obtained by interpolation. The amount of shift at the true maximum value is considered as an amount of displacement.
Since an optical system has a unique relationship between the amount of displacement and the amount of movement of an image surface, that is, a so-called defocus amount, the amount of defocus is determined from the amount of displacement. An amount of movement of the focus lens is found from the defocus amount, and the lens is moved to achieve focusing.
As described above, the AF of the phase difference detection scheme detects a defocus amount for a subject, so that it is possible to determine in which direction the focus lens should be driven by what amount to achieve focusing by calculations based on the detected defocus amount. Thus, achieving focusing requires lens driving only once, and fast, quiet and accurate AF control can be performed while the characteristics of the vibrating type motor are made use of. For this reason, a number of commercially manufactured cameras or interchangeable lenses are equipped with the phase difference scheme AF and the vibrating type motor.
On the other hand, in a digital camera which acquires images by a two-dimensional image pickup device and electrically records an image signal on a recording medium, an AF scheme referred to as a contrast detection scheme is employed.
The contrast detection scheme AF generally operates as follows. The configuration thereof has an image-taking system including a two-dimensional image pickup device, a system control section including a calculation circuit and a circuit for producing a control signal for driving a focus lens, and a lens section including a lens control circuit for moving the focus lens in an optical axis direction.
The image-taking system admits image light, and outputs and sends it as an image signal to the system control section which in turn extracts higher frequency components included in the image signal. The maximum value of the extracted signal is stored. The focus lens is moved by a certain amount in a certain direction. Then, image light is again admitted and higher frequency components are extracted.
When the maximum value of the extracted signal is larger than the previously stored value, the focus lens is considered as moving toward an in-focus position. The current value is newly stored and the focus lens is moved in the same direction.
When the current maximum value of the extracted signal is smaller than the previous one, the focus lens is considered as moving away from the in-focus position. The current value is newly stored and the focus lens is moved by a certain amount in a direction opposite to the previous moving direction. Then, image light is again admitted, higher frequency components are extracted, and the maximum value is compared with the newly stored one. The image surface is finally caused to reach the in-focus position.
Description is made with reference to FIG. 15. The horizontal axis of the graph in FIG. 15 represents the position of an image surface and the vertical axis represents the maximum value of higher frequency components. A point a indicates the position of an image surface at a starting point and a point b indicates the in-focus plane. The maximum value of higher frequency components at the point a is assumed as “A.” The focus lens is then moved to the right in the graph, that is, in a direction toward the in-focus plane. The maximum value of higher frequency components at a point a′ after the movement is “A′” and a comparison between them shows that A′ is larger than A. The focus lens is thus continuously moved in the same direction.
After several comparisons, A is larger than A″ (A″ is the maximum value at a point a″) at an image surface position past the point b, and it is possible to determine that the focus lens is now moving in a direction away from the in-focus plane. Thus, the moving direction of the focus lens is reversed to match the image surface to the in-focus plane.
As described above, since the contrast detection scheme AF involves movement of the lens toward the in-focus position while searches are made for the lens position where higher frequency components of the image obtained from the image signal are at maximum, image signals need to be acquired with the position of the focus lens being moved gradually. Thus, the lens driving requires repeated driving over a short distance. To reduce a time taken for achieving focus, the repeated driving and stop of the focus lens must be performed quickly.
When the aforementioned contrast detection scheme AF is used in a digital camera, it is necessary to perform driving of a focus adjusting lens by a small amount and AF operation a number of times to bring the focus adjusting lens near the in-focus point if an image-taking lens has a large focal length and an image on an image pickup device is significantly blurred, which presents a problem of a long time taken for the focusing operation.
In contrast, the phase difference detection scheme AF does not present such a problem since focus can be substantially achieved by one-time lens driving.
On the other hand, a digital camera has an image pickup device significantly smaller in size than a silver film and thus requires a higher resolution for AF detection corresponding the ratio of the sizes. The phase difference detection scheme AF, however, has a problem that this requirement cannot be satisfied due to the limitation on the size of the secondary imaging lens 508 shown in FIG. 12.
In view of such problems, a so-called hybrid scheme AF has been proposed in which the phase difference detection scheme is first used to perform focusing operation of a focus adjusting lens at low resolution and then the contrast detection scheme is used to perform fine adjustment of the focusing operation.
The vibrating type motor, however, is not sufficiently excellent in responsiveness upon actuation for performing the contrast detection scheme AF which requires quickly repeated driving and stop although the vibrating type motor has the characteristic of high responsiveness to some extent. A long time may be needed before focus is achieved in the contrast detection scheme AF.
In the vibrating type motor, the vibrating member is in strong frictional contact with the moving member, and the frictional contact is used to transfer a driving force. Thus, to move the moving member in a traveling vibration wave at the time of start of driving, a large force exceeding the static friction force during stop of the moving member is required, which causes insufficient responsiveness upon actuation.
To address this, proposals for improving the responsiveness upon actuation have been made in which the vibrating member of the vibrating type motor is provided with vibrating energy before the start of driving with a traveling vibration wave by causing the electro-mechanical energy conversion elements to produce a standing vibration wave and then a traveling vibration wave.
Among the proposals, Japanese Patent Application Laid-Open No. 8-80073 proposes a method of driving a vibrating type motor in a standing wave switched from traveling wave driving for a certain time period after an in-focus state is reached.
In the contrast detection scheme AF, however, the remaining driving amount cannot be calculated due to its characteristic that the in-focus position is searched for while the lens is driven little by little as described above, and thus the technique in the aforementioned proposal cannot be employed. In addition, the aforementioned proposal attempts, after completion of focus adjusting operation of a focus lens (after focusing is achieved), to improve startup characteristics of the next focus adjusting operation, and does not attempt to reduce a time taken for operation before focus is achieved.